TRAFFIC COLLISION INVESTIGATION MANUAL FOR PATROL OFFICERS _______________________ LUIS E. MARTINEZ ACTAR #38 SECOND EDITION

Introduction This Traffic Collision Investigation Manual for Patrol Officers is a replacement for a short pamphlet written by the author in 1985 for use at the Central Arizona Regional Law Officers Training Academy (CARLOTA). The original booklet was also used with success for in-service training of police officers. The new and updated second edition has been expanded to include additional information on vehicle dynamics, speed calculations and equations, and diagraming methods. It is a helpful reference text to any police recruit collision investigation course. The manual is also useful as an entry level introduction to collision investigation for non-police personnel, such as attorneys and insurance investigators. About the Author Luis Martinez is a police officer with a central Arizona police department and has been in law enforcement since 1979. He started teaching collision investigation at CARLOTA in 1982. Sgt. Martinez is also an adjunct faculty member at Central Arizona College, teaching criminal justice courses. He has completed courses in collision investigation and reconstruction from The Traffic Institute (Northwestern University) and the Institute of Police Technology and Management (University of North Florida). He is an accredited reconstructionist by the Accreditation Commission on Traffic Accident Reconstruction (ACTAR), and has testified as an expert in accident reconstruction in municipal and superior courts. Luis holds a bachelor of arts degree in management from the University of Phoenix and is currently attending graduate school (Northern Arizona University), working on a Master of Education degree. SECOND EDITION Copyright 1994 © by Luis Martinez

PERMISSION IS HEREBY GRANTED TO REPRODUCE THIS MANUAL FOR TRAINING PURPOSES ONLY, SO LONG AS THE COPYRIGHT NOTICE IS RETAINED. REPRODUCTION FOR COMMERCIAL PURPOSES IS NOT AUTHORIZED. ________________ All Rights Reserved.

ii

TABLE OF CONTENTS Page Introduction................................................ i Chapter 1: Answering the assignment call.................... 1 Patrol vehicle operation.................. 1 Collision scene management................ 3 Traffic cones/flares...................... 7 Interviewing drivers & witnesses.......... 11 Chapter 2: What the road shows.............................. 16 Tire marks................................ 18 Skid marks................................ 20 Skip skids................................ 23 Gap skids................................. 24 Acceleration marks........................ 25 Tire tread print.......................... 25 Scuff marks............................... 26 Metal scars............................... 27 Debris.................................... 31 Other signs of impact..................... 32 Chapter 3: What the vehicle shows........................... 36 Contact and induced damage................ 36 Exterior damage assessment................ 38 Vehicle impact orientation................ 40 Rollovers................................. 42 Vehicle interior examination.............. 43 Chapter 4: Measuring and diagraming the collision scene..... 46 What to mark and measure.................. 46

iii

Diagraming the scene...................... 51 Coordinates............................... 52 Triangulation............................. 56 Drawing the scale diagram................. 59 Chapter 5: Calculating speed from tire marks................ 76 Speed from skid marks..................... 76 Test skids................................ 80 Drag factors from published tables........ 83 The template nomograph.................... 84 Critical speed (yaw) marks................ 85 Combined speeds........................... 88 Chapter 6: The collision report narrative................... 92 Example #1................................ 93 Example #2................................ 95 Appendix Equations for the traffic collision investigator...... 99 Traffic collision terminology......................... 102

iv

Chapter 1: ANSWERING THE ASSIGNMENT CALL

An officer's responsibility to conduct a motor vehicle

collision investigation begins at the time he or she is

directed to the collision scene. It is at that precise

moment that the officer should begin to obtain as much

information as it is available from the radio operator, such

as : type of collision, location, extent of injuries,

traffic condition, etc1.

PATROL VEHICLE OPERATION

Most, if not all, law enforcement agencies have in

place policies and procedures that determine police response

based on the seriousness of the service call. Thus response

is generally classified as : routine, urgent, and emergency.

In situations that require an emergency response (use of

lights and siren), it is imperative that the officer drives

to the scene practicing good emergency driving techniques.

Considerable time and effort is currently invested at

every police academy towards the goal of producing safe and

effective police drivers, yet, police files are full of

cases where these safe techniques were either not utilized

1 This type of advanced planning is useful in avoiding the tunnel vision effect so common when responding to high stress

calls.

2

at all, or applied improperly. The results of these failures

are wrecked police and civilian vehicles, painful injuries

and, sadly, tragic deaths.

It is of use to no one if a responding police vehicle

is itself involved in a traffic collision. A secondary

(police involved) crash simply complicates the situation.

Many of the collisions involving police vehicles in an

emergency response mode can be prevented.

The very first steps an officer should always take

during this kind of high stress response is to simply: take

a deep breath, remain calm, and recognize that this is

nothing out of the ordinary but, in fact, a type of call the

officer has been trained for and is quite capable of

handling. Self-relaxation techniques, such as deep

breathing, are effective and will help any officer when

dealing with stressful calls, not just collisions.

The subject of what should be an appropriate driving

speed when enroute to an collision is controversial, and

difficult to define. Some law enforcement agencies establish

strict guidelines on speed limits, others leave that

decision up to each individual officer. It would be

difficult and purely arbitrary to establish absolute speed

limits within the context of this discussion, without

consideration to various factors: individual driving skills,

equipment, terrain and weather conditions and highway

3

design. What can be stated without hesitation is that each

officer must consider each one of this factors before

deciding what a safe speed will be for that particular

emergency run.

One of the factors mentioned above is really only known

to each officer, that of individual driving skill, and only

he/she can decide when those skills are being tasked to

their limit. Unfortunately a large part of the "police

myth", like the belief that "cops are the best shooters", is

the notion that "all cops are Mario Andretti" and can

perform driving maneuvers beyond the capabilities of mere

mortals. When only civilians believe these things, no real

harm is done...when police officers themselves begin to

think like that, trouble is not far away.

The bottom line on the speed issue is simple: officers

must (a) know the limits of their driving skills, and (b)

stay within those limits. The time lost when reducing the

response speed by 10 or 15 miles per hour is insignificant

in most cases, and can mean the difference in getting there

in one piece, or not at all.

SCENE MANAGEMENT

Of course, all of the above discussion can simply be

expressed in one phrase: having a plan. A professional

4

police officer should never fail to plan ahead and have a

strategy for action when responding to any call, not just

traffic collisions. Since all collision situations are

different the officer must also be flexible in the advanced

planning and be prepared to adjust or change tactics based

on the given scenario.

Upon arriving at the collision scene a primary concern

should be preventing the collision from getting worse, that

is, effective scene management. Traffic channelization

should be used to avoid second and third collisions.

Motorists are strongly attracted to collision scenes and

"gawking" is the primary cause of a great many secondary

collisions.

At the early stages of the process we refer to as scene

management, the officer's primary tool to control traffic

and to protect those persons injured is the patrol vehicle.

It should be parked on the roadway and in such a way as to

provide a shield between the crashed vehicles, the injured,

the officer, and traffic. Obviously the emergency overhead

red-blue lights should be operating and visible to oncoming

traffic.

A common mistake after arrival at the collision scene

is leaving the patrol car's trunk lid opened after obtaining

items from the trunk. A raised trunk lid will obstruct the

overhead lights and make them invisible to approaching

5

traffic. Some departments have addressed this problem by

installing a second set of red lights inside the vehicle's

trunk lid, they come on automatically whenever the trunk lid

is raised. Lacking a system like this one, the best

alternative is to always remember to close the trunk lid.

6

Figure 1 on the preceding page shows the proper

positioning of the patrol vehicle at the collision scene in

order to provide the best protection to the officer and

Figure 1- Recommended patrol vehicle position at the collision scene

7

civilians alike. The police vehicle is depicted in black,

the collision vehicles are outlined in white2.

Nighttime traffic collision scene management presents

an added set of safety factors not present during daylight

hours. Visibility is severely restricted and the large

number of emergency vehicles at the scene, with overhead

lights on, tend to confuse and "hypnotize" approaching

drivers. This common phenomena causes the driver visual

attention to "home in" on the lights, and thus fail to see

other things on the roadway directly ahead (such as police

officers diving out of the way). Whenever possible the

officer should work while facing the oncoming traffic.

The use of reflectorized safety vests is an absolute

must when working nighttime collisions, particularly if an

officer's duty uniform is of the dark or black variety. If

the law enforcement agency does not issue this type of vest,

it is a worthwhile investment for any patrol officer to

obtain one of these vests.

On minor traffic collisions (fender-benders) the

officer would be well advised to quickly mark (see Chapter

4) the position of the vehicles and remove them from the

roadway, restoring traffic flow and enabling the officer to

continue his investigation out of harm's way.

2 In this position the police vehicle serves as a shield to protect the officer against drivers not alert, the impact noise

of a collision with the patrol car would warn the officer to impending danger.

8

TRAFFIC CONES/FLARES

Although the patrol vehicle is the first available tool

to use in traffic control, devices such as plastic orange

traffic cones and flares should not be overlooked. They

serve as long range channeling devices when the clearing of

the roadway will not be immediate. As soon as the first

responding officer has assessed the injuries involved and

required medical assistance is on its way, he/she should

begin to establish a traffic corridor to divert cars from

the scene and avoid a traffic bottleneck at the location.

The orange cones are a useful item to carry in the

cruiser's trunk for daylight use, however, their size and

bulk limit the number of them that can be carried to no more

than 8 to 10 average size cones. Flares, on the other hand,

are compact and the average police car can carry 2 boxes

with room to spare.

CAUTION- Before igniting any flares the officer must

insure that no gasoline leaks or other flammable materials

are present. Many tractor-trailers involved in interstate

commercial transportation carry flammable and/or toxic

materials, exposure to fire could be catastrophic.

When utilizing cones or flares to establish a traffic

control pattern, the key thing to remember is: how does the

9

traffic control devices appear to an approaching motorist ?

Officers sometimes throw a few scattered flares on the

roadway and expect oncoming drivers to somehow interpret a

pattern and follow a proper path, where there is none. A

lane closure requires the placing of flares at evenly spaced

distances in a pattern that conveys a message to the

motorist, mainly which direction are motorists expected to

go...or not to go. This is accomplished by placing the

flares lined at an angle leading away from the roadway edge,

and in the direction traffic is expected to flow.

10

Figure 2- Suggested method to channel vehicular traffic using flares or orange cones, or to block the entire roadway

11

Figure 2 shows a recommended pattern to use in

redirecting traffic flow (A) when the collision is confined

to one lane. This will require another officer to control

the flow of traffic in each direction. If the complete

roadway is obstructed (B) then the officer must close the

entire roadway width.

Officers must keep in mind that the distance gap

between the first flare or cone positioning and the

collision scene is dictated by the speed at which the

majority of the traffic is traveling. Motorists must be

given a reasonable

amount of time to

recognize the problem

and react

appropriately. Table 1

shows a recommended

safe distance for the

proper placement of

flares or cones. A motorist should be able to stop before

arriving at the collision scene, even if he/she does not see

the cone/flare until arriving at the first one. This table

is based on the stopping distance of an average passenger

vehicle on dry asphalt and using a reaction time of 1

second. Obviously, if the roadway is wet or a driver is

Speed Limit 1st Cone/Flare 25 mph 68 feet 35 112 45 167 55 227 65 301

Table 1

12

fatigued or under the influence of alcohol the reaction time

is going to be longer.

A distance gap of 60 to 70 feet (from first warning

device to collision scene) might be adequate in a slow speed

zone (25 MPH), 300 to 400 feet may be required on a major

state highway or under severe weather conditions. Officers

must utilize extreme caution when on a traffic way and

always remain alert and facing traffic whenever possible.

If the roadway is wet or iced over, the safe distances

above will not be sufficient and a traffic control point

manned by an officer 1/4 to 1/2 mile ahead will be

necessary. Nighttime collisions, highway curves, and hills

present added safety problems during scene management and

the officer must consider the visibility problems they

create.

The method described above is really only applicable

when the colliding vehicles are confined to one lane of

traffic on a multi-lane (2 or more lanes) roadway. If the

collision scene occupies the entire width of the roadway,

channelization will not be effective as there is no place to

channel traffic to. In such a case it would be better (and

safer) to completely close the roadway to all traffic until

the collision scene has been cleared.

INTERVIEWING DRIVERS AND WITNESSES

13

The immediate purpose of any traffic collision

investigation is to obtain the relevant facts about the

incident. The level of effectiveness in a criminal

investigation is largely dependent upon the investigator's

ability to obtain information. In an collision investigation

this ability is no less critical. It is important to

differentiate, at this point, interviews from

interrogations. The balance of this discussion will address

the process of interviewing drivers and witnesses in a

traffic collision.

It does not cover the interrogation of a driver

suspected of a criminal violation, i.e., driving under the

influence, and the required legal and constitutional

guidelines. The reader is referred to other textbooks on

criminal interrogation for such information.

Upon completion of the preliminary traffic control

function described above, the patrol officer's next priority

is to locate the drivers of the vehicles involved in the

collision. They should be located as soon as possible and

identified through their driver's licenses, which should be

kept by the officer until the investigation is completed.

Securing driver licenses safeguards against a driver leaving

the scene before the investigation is completed. As an added

14

method of identification each driver should be asked to

point out the other.

Driver interviews must be conducted individually, with

each driver given the opportunity to state what happened

away from the other driver. This will prevent argumentative

behavior, and it will keep the second driver from

"developing" defenses while listening to the first

driver.During the interview it is important to closely

observe each driver for signs of alcohol or drug use,

injury, or shock.

Disinterested parties (bystanders) who were not

involved in the collision are generally the most effective

and reliable eyewitnesses. They should be contacted as soon

as possible by the investigating officer, or any other

available officer. It should be remembered that bystanders

are attracted to the excitement and atmosphere of the scene.

Once this begins to dissipate, so will they.

Sometimes managing the scene will take all of the first

responding officer's time, with little or no time left for

interviewing witnesses and obtaining statements. The

investigating officer may have to limit his inquiry to

obtaining name and addresses only, and interview these

persons at a later date. Injured drivers/passenger may have

to be removed from the scene quickly. Follow up interviews

may have to be conducted at the hospital or at their homes.

15

When interviewing witnesses an officer must be

particularly careful and very specific. Questions such as:

"Did you see what happened"? will only elicit vague or non

responsive answers. Be specific: "Where were you standing?

Where you looking at the car when it collided with the tree?

Was your attention attracted by what you saw..or what you

heard? Did you hear the crash first...and then looked"? A

good idea when interviewing witnesses and drivers is to have

them reenact what they saw, at the actual scene of the

collision. If this is not possible, use pen and paper and

have them draw what they saw, such as vehicle positions and

movement.

Whenever possible, the use of written statements is

preferred over notes taken by the officer. After obtaining

written statements, the officer should read them and clarify

any issues not clear. Do not overlook the value of

interviews in traffic collision investigation.

16

NOTES ____________________________________

17

NOTES ____________________________________

18

Chapter 2: WHAT THE ROAD SHOWS

The investigation of a traffic collision, whether it is

a property damage-only collision, one involving many

injuries, or the dreaded fatal, is pretty much like

investigating any crime. Some officers who spend hours and

spare no resource to asses and process a crime scene will

attempt to "speed" through an collision scene investigation,

with disastrous results. Unfortunately this is not realized

until six months or a year later, when issues arise

concerning the investigation. It is then that a hapless,

overpaid (civilian) or underpaid (police) reconstructionist

is handed an incomplete report, with important roadway

evidence missing, and asked: "Tell me what happened !" One

of the author's career long objective and teaching goal has

been to put forth a strong but simple message to police

recruits: traffic collisions involving injuries/fatalities

are crimes, in the strictest definition, and should always

be treated as such.

Some police personnel have chosen the position that

collision reports are only of benefit to attorneys and

insurance companies and they are not "real police work".

Nothing could be further from reality. The primary mission

of any law enforcement agency worth its title is to serve

and protect the public. Collision investigation and

19

collision reconstruction certainly fall under the "to

protect" definition. If third parties gain some benefit from

our investigation, incidental to our primary purpose, then

so be it.

Police investigators do not normally approach a crime

scene expecting the victim(s), particularly if he/she is

deceased, to tell them much of anything. The traffic

collision investigator should approach the collision scene

in the same manner. This is not to imply that witness

testimony is not important (see Chapter 1) but to underscore

a point: sometimes there are no witnesses and the victims

are in no position (or disposition) to provide any useful

information as to what occurred.

In such a situation the evidence left on the roadway,

and the officer's interpretation of said evidence, become

crucial in the final determination of exactly what happened.

Roadway evidence can provide the investigating officer

useful information, when located and properly interpreted.

This physical evidence from a crash can be used to confirm

the final resting location of each vehicle, the direction to

which each vehicle faced at final rest, and the path the

vehicle followed from impact to rest. Final resting

locations are generally classified as controlled and

uncontrolled.

20

Controlled final resting locations are those which a

vehicle reached with direct human intervention after the

initial impact. An example of this would be a driver who,

after exiting his vehicle to ascertain the extent of the

damage, reenters the vehicle and parks it at the edge of the

curb of the road to wait for the police. This type of

controlled final resting location means little to the final

determination of collision angles and post-collision paths

and is of little significance to an collision investigator.

An uncontrolled final rest location is that which a

vehicle (or a pedestrian) reaches without direct human

intervention, that is, human control is deemed to cease at

the instant of collision. A pedestrian struck by a car who

is vaulted a certain distance, lands and does not move again

by his own volition is a good example of an uncontrolled

final rest (no pun intended!) location. It is this type of

positioning that is most significant to the investigator and

reconstructionist in determining departure paths and

post-collision velocities.

The roadway evidence helpful in determining vehicle

positioning and paths consist of : tire marks, roadway scars

(scrapes, gouges), debris, markings that suggest the vehicle

went airborne, and markings to other objects on or near the

roadway. Whenever possible, and always in collisions

involving death or serious injury, photographs of all

21

roadway evidence should be obtained. Photographs should be

taken prior to the placement of marks on the roadway (chalk

or crayon).

TIRE MARKS

Tire marks and roadway evidence are extremely important

in determining how vehicles moved in to the impact point,

and from impact to final resting locations. In collision

investigation tire marks are the equivalent of a fingerprint

in a criminal investigation, and just as important.

Before we can properly analyze tire marks we must first

understand how they are made and the braking mechanism

involved. The majority of the passenger cars on the road

today still use the standard hydraulic braking systems for

their primary braking systems, and mechanically activated

brakes for their parking brakes.

When brakes are applied the vehicle has a tendency to

shift forward, this results in a weight transfer from the

rear wheels to the front wheels. Typically, 55% - 60% of the

total weight of the vehicle rests on the front wheels during

braking, leaving 45% - 40% weight applied to the rear

22

wheels. There are complex weight-shift equation that can

provide an exact percentage of weight shift for a given

vehicle, however, they are a bit more complex and beyond the

scope of this handbook.

When the brakes are applied, the kinetic energy of

motion (as a result of the vehicle's mass times velocity) is

transformed into heat by friction. If the brakes are applied

strongly enough the tire itself will lock. This is sometimes

referred to as 100% slip. The kinetic energy, or energy of

motion, is then dissipated between the tires and the roadway

surface in the form of heat.

It is this heat that dissolves, or melts, the tars and

oils on the roadway surface, thus creating the distinctive

dark smear we commonly refer to as a tire mark, or in this

case, a skid mark. Although some small particles of tire

rubber do separate from the tire itself, the skid mark is

primarily composed of asphalt tar.

On concrete surfaced roads skid marks are lighter in

color. They are made by the rough concrete surface actually

"grounding up" the tire, or melting it. Sometimes the

"squeegee" effect of the tire will actually clean the dirty

road surface, resulting in a skid mark lighter in color than

the surrounding surface. When a vehicle travels with its

tires locked (sliding) through a soft or loose surface it

23

will plow through the loose material, pushing it out to the

sides and ahead of the tire.

SKID MARKS

This skid mark is created by a tire that is locked,

that is, sliding and not rolling. Skid marks tend to be

straight, although they can exhibit some curvature due to

asymmetrical braking (not all brake pads locking

simultaneously) or due to the crown of the road. This can

make the vehicle depart from a straight-ahead path. Front

tire skid marks tend to be darker than rear tire marks

(remember weight shift) and the outside edges of the mark

may be darker than the inside area, due to over deflection

of the tire (weight shift, again). The tire grooves are

generally visible and easy to see in a skid mark. Rear tire

skid marks tend to be even in appearance, that is, no dark

outside edges.

Skid marks are an extremely important piece of physical

evidence to the collision investigator. They can be used to

determine speed (see Chapter 5) and to establish the path of

the vehicle while skidding. Unfortunately, tire mark

evidence has a life span. They are affected by weather,

sunlight, and traffic.

24

Tire marks can be obscured by movement of other

vehicles at the collision scene (very common on gravel

roads), and destroyed by the ever present evidence

eradication team, sometimes known as the fire department!

Therefore, it is most important that tire marks be located,

measured, and properly documented before their

disappearance.

When looking for skid marks it is of extreme importance

to determine the point where the skid mark begins. This

beginning point of the skid mark is a relatively faint mark,

as compared to the rest of the dark tire mark. During

initial brake application there is a short time delay

between the time the braking system/tire combination locks

the wheel, and the point at which the tire heats up

sufficiently to begin leaving a mark.

This faint beginning is called the skid mark shadow. To

locate this shadow the investigator must kneel or bend down

to the roadway level, 20 to 30 feet ahead of the mark, and

look towards the apparent beginning of the skid mark. A

second person is required to assist the investigator in

marking the beginning point of the shadow with a crayon or

paint.

The location and inclusion of this faint beginning of

the skid mark is extremely important for future use in speed

determination. Testing by the author shows that as much as

25

10 percent of a skid mark can be in this shadow, overlooking

it can underestimate the speed of the skidding vehicle

considerably.

Curved skid marks indicate that the vehicle that made

them is rotating while simultaneously skidding. When this

happens all four tire marks can be observed. This rotation

during a skid can be initiated by the driver beginning a

turning maneuver as wheels are locked in braking, the

vehicle then continues in rotation.

Curved skid marks can also be indicators of unequal

braking. If left side tires are braking with greater force

than right side tires, the vehicle will tend to rotate

counterclockwise in the direction offering the higher

resistance.

Skid marks that are curved can also be indicative of a

half spin, this is a rotation of the vehicle 180 from its

original direction of travel. This is caused by the rear

tires locking up before the front wheels. A vehicle is less

stable in terms of directional control when rear tires are

locked up before the front ones. Rear tires will then loose

the necessary lateral forces which are essential to

directional control of the vehicle. When this happens the

vehicle will "switch ends."

If there is a question as to the operational status of

all or any brakes, an examination of every tire should be

26

conducted. This will reveal the presence, or absence, of

abraded areas at the road/tire interface, also known as skid

patches.

SKIP SKIDS

When skid marks are not continuous but are

intermittent, they may have been made by a vehicle bouncing

along on the roadway. In this situation the length of the

skid mark and the length of the space between them is

uniform and consistent, and less than 3 to 4 feet apart.

This condition can result when the wheel strikes a pothole,

or bump on the roadway, which starts the vehicle bouncing.

Skip skids should be measured for total length so that

the gaps are included in the finished measurement. These

gaps are considered to be a part of the overall skid mark.

Vehicle braking is not reduced during the skip portion of

the skid by virtue of the wheels being off the ground for

such short distances.

While actual braking does not occur during these short

intervals when the wheel leaves the ground, heavier braking

does occur when it returns back to the ground to compensate

for the missing distance. This effect has a tendency to

average the energy lost and results in valid speed

estimations from skip skids. Skip skids, like any tire mark,

27

should be documented accurately as well as photographed for

further evaluation of the skid mark.

GAP SKIDS

Many times skid marks are observed in which there is a

gap between the termination of the skid marks on the roadway

and a re-initiation of the skid mark some distance down the

roadway. This is the result of the driver applying the

brakes, and subsequent release and re-application. Sometime

the driver may momentarily release the brakes because he/she

believes that the collision conflict situation ahead, in

which a collision appears imminent, has passed, only to re-

apply them again when he/she realizes that the initial

judgement was incorrect.

This is a typical situation in collisions with

pedestrians or bicyclists, where the slower movement of the

person or bicycle can change suddenly from that which the

driver anticipates. Unlike skip skids above, such gaps in

skid marks are not included in the overall measurement of

the skid mark. Gap skids are measured separately, as if they

were made by two separate vehicles. A combined speed

approach is then utilized in calculating a speed. The

chapter on speed calculations presents the mathematical

approach for this situation.

28

ACCELERATION MARKS

Often, tire marks that look like skid marks are really

acceleration marks. These marks are created when a vehicle

is accelerated rapidly from a stopped position or when

moving at a slow speed, so as to make dark tire marks. These

acceleration marks closely approximate skid marks in their

appearance. They begin as heavy dark marks and slowly

disappear as the rotational velocity of the tire begins to

approach the linear velocity if the vehicle.

One characteristic common to acceleration marks is

their linearity. When rapidly accelerating a vehicle under

forward maximum acceleration, some steering is necessary to

maintain the path straight. This is because the torque from

each wheel may not be equal at each rear tire because of

road-tire interface differences3. This small difference must

then be corrected by steering. This results in a curved or

"wavy" appearance.

TIRE TREAD PRINT

An imprint of the tire tread pattern indicates that the

wheel was rolling and not skidding. The effect thus created

3 For front wheel drive vehicles this would then be true of the front drive wheels.

29

is much like an ink stamp, in which the pattern of rubber is

imprinted on a flat surface without smearing. The print may

be the result of loose matter picked up by the tire as it

rolled on the roadway.

Tire prints are different from skid marks in that they

convey the tire tread pattern of the tire without any of the

slick or smoothly worn features characteristic of a skid

mark. In addition, the print pattern is uniform in contrast

and noticeably similar to other print marks left by tires on

other wheels of the same vehicle.

SCUFF MARKS

Scuff marks, also known as yaw marks or critical speed

scuffs, are tire marks left on the roadway by wheels that

are sliding and rolling simultaneously. The wheel is rolling

and slipping sideways at the same time.

When a vehicle "spins out" or "slips out" while

cornering, or is oriented in a direction different from its

direction of travel, scuff marks will be deposited. Often

they are in the form of light parallel grooves, referred to

as striations or hash marks, which run straight but are

diagonal to the outline of the continuous scuff mark. They

are made by the sidewall or rib of the tire.

30

One of the most important bits of information one can

obtain from scuff marks is that the vehicle is taking a turn

at a critical cornering speed. Critical cornering speed is

the speed at which the vehicle is on the threshold of

spinning out or slipping laterally.

Tire scuff marks which occur under these circumstances

are critical speed scuffs made by tires sliding as the

vehicle traverses the curve, and are made by the outside

edges of the tires. The scuff mark left by the rear tire

will fall outside the scuff mark made by the front tire for

that side of the vehicle tending to slip off the roadway as

a result of centrifugal force.

31

Figure 3- Typical curved scuff mark appearance

Figure 4- Close-up view of scuff mark, showing the striations

32

It is important to remember that scuff marks are made

by steering, or rather over-steering, as opposed to skid

marks which are made by braking. Two important

characteristics to look for when examining scuff marks are

their curved path and the striations (see pages 28 & 29).

METAL SCARS

When a moving vehicle is damaged in such a way that

metal parts come in contact with the roadway surface, scars

or scratches are left. Scars are helpful in indicating the

direction of movement of the vehicle on impact. When they

are correlated with the parts of the vehicle which made the

scars they can also confirm the position of the vehicle on

the roadway at impact.

Scars resulting from rollovers may indicate where the

vehicle initiated its rollover movement. A vehicle sliding

along the pavement on its side, or top, leaves distinctive

scratches made by sharp sheetmetal edges or other protruding

parts. Scars may also indicate the direction of impact as

well as the relative force of the impact. This is true when

the scar can be matched with the undercarriage portion of

the vehicle (engine, frame, transmission, differential,

etc.) that made the scar.

33

There are also some instances in which scratches on

pavement prior to impact are indicative of a failure of some

vehicle component. An example of this is when scratches

occur from the rim of a wheel which sustained a flat tire

before the crash. This can be confirmed by the distinctive

pattern of a wobbly flat tire as it moves across the

pavement.

Gouges may be distinguished from scratches in that they

are much deeper and wider, and tend to chip or chop chunks

of road surface material. Examination of the undercarriage

of the vehicle can indicate abraded areas which may have

gouged the pavement. Deep gouges are characteristic of

severe head-on collisions, where the front end of one or

both vehicles are driven down into the road surface with

tremendous force. These deep road gouges are good indicators

of the point of impact, or area of collision.

DEBRIS

Debris is not necessarily related to surface effects

made by the vehicle directly in contact with the surface

over which it is moving. However, inasmuch as debris

34

associated with the collision is physical evidence similar

in importance to other crash evidence, it should be

considered with the same careful attention. Debris are any

large or small vehicle parts or pieces separated from the

vehicle as well as undercarriage dirt and mud which drops

onto the roadway, rust from metal parts which is shaken

loose, paint, various vehicle fluids (radiator coolant,

engine oil, brake fluid, battery acid, etc., and similar

materials. debris may also consist of portions of the

roadway which are scooped up and spread in the collision

area.

In severe injury producing crashes, debris may include

human fluids and other matter, as well as portions of human

bodies. Blood is perhaps the most common here, but may also

be accompanied by other human matter. Pools of blood can

show where an injured person lay or crawled to or was moved

to after the crash.

Material carried by the vehicle, or within its

passenger compartment, trunk or cargo area, is often

dispersed about the crash site. This is an important

consideration when attempting to interpret debris associated

with only one crashed vehicle. Fluids are of particular

significance. When a fluid container bursts, the fluid is

dispersed with the same velocity as the vehicle. This

35

results in fluid patterns which can help determine vehicle

movement at impact.

Debris in the form of fractured parts which separate at

impact are deposited in a pattern consistent with the

dynamics of the crash. This can be helpful in understanding

the collision. Some parts can travel great distances, due to

their relatively light mass and great initial velocity. In

head-on collisions, for example, parts from each vehicle can

be propelled forward a considerable distance from the impact

point.

OTHER SIGNS OF IMPACT

Damage to fixed objects, such as bent and broken

guardrails, posts, trees, and other fixed objects can give

some idea of the speed and position of the vehicle striking

them. Such damage is best described by photographs. When

matched with damage or marks on the vehicle they can fix its

position at a specific point in the chain of events.

Sometimes particles of paint in the scraped area will help

distinguish which of the vehicle involved in the collision

came in contact with that object.

The term fall is used to describe a vehicle which has

left the ground for a short time, while falling or flipping.

Signs of this are found at the beginning and particularly at

36

the end of the fall, in the shape of marks on the ground

itself. Flips or vaults occur when the moving vehicle

strikes an obstruction that suddenly stops the wheels. The

vehicle then pivots upward and leaves the ground, usually

landing bottom up. The signs of tire marks at the take-off

and landing points should be carefully located. If the

vehicle was sliding in soft material the tires will dig a

furrow, this is a distinctive spot and very easy to locate.

A vault is an endwise flip.

37

NOTES ____________________________________

38

NOTES ____________________________________

39

Chapter 3: WHAT THE VEHICLE SHOWS

Now that the road markings and the information they can

reveal have been discussed, we can turn our attention to the

technical examination of the vehicle.

Final position of the vehicles is usually the most

important observation to make on all collisions. A quick

glance at all traffic units involved upon the officer's

arrival can give a general idea as to final positions. This,

of course, should be followed by efforts directed at

locating the vehicles by measurement.

40

Figure 5- Standard photographic record (4-view) of vehicle damage

41

If photos are to be taken, the basic four view sequence

of photographs should be taken before allowing vehicle

removal (page 37). Additional photos should be obtained

showing closeup or detailed areas of damage, matching damage

parts of two vehicle showing how they came in contact, and

any other item on the vehicle that can help explain the

collision.

CONTACT AND INDUCED DAMAGE

Before any discussion on vehicle examination can begin,

an explanation of the terms contact and induced damage is in

order. Contact damage is damage to any part of the vehicle

caused by direct contact with some other object. This object

can be another vehicle, a pedestrian, a fixed object, or

even the road surface itself.

External contact damage is shown by rubbed-off paint

(paint transfer), crumpled vehicle skin, tire rubber, road

material, tree bark, and even human tissue or clothing. It

is also indicated by imprints of headlight housings, wheel

rims, bumpers, door handles, poles, and other fixed objects.

Induced damage is damage to any part of the vehicle

caused by the forces of the collision. Damage to the

42

differential or universal joints, for example, is induced

damage because these parts are not broken by direct contact.

Contact damage to vehicle glass has specific effects

depending on which type of glass is involved. On the safety,

or laminated, glass used in the front windshield contact

damage leaves fracture lines radiating away from the contact

point. These circular lines formed what has been described

as a "spiderweb" pattern. Parallel fractures (checkerboard

pattern) on a front windshield are indicative of induced

damage. Tempered glass, used in side and rear windows,

display no pattern when subjected to either contact or

induced damage, it simply shatters.

EXTERIOR DAMAGE ASSESSMENT

The second step taken by the investigator, after

assessing final positions of the vehicles, should be to

determine if the damage present is consistent with his/her

understanding of the crash in terms of vehicle orientation

and dynamics. Many collisions which are listed as single

impacts will actually involve more than one impact, i.e.,

the vehicle bounce after impact into a fixed object, or

vehicles may rotate quickly after impact and contact each

other a second time. Crash damage to the vehicle is helpful

43

in determining how the object was struck. This is most

easily observed on impacts with narrow objects.

In vehicle-to-vehicle impacts the investigator should

look for damage that matches both vehicles. this can be like

a jigsaw puzzle, in which the contour of one piece must fit

the contour of another. Ejection avenues should be noted and

listed in the report. These may be any opening in the

vehicle through which a human body may pass (side glass

openings, windshield openings, opened doors, etc.). These

ejection portals may not be large, the human body is very

pliable and can take various shapes and postures.

Gross vehicle failures in crashes should also be noted.

These could be in the form of a large rupture, such as

separation of the roof, doors, and other structural damage

which renders the passenger compartment useless as a secure

area for occupants.

Damage to wheels and tires of vehicles also require

careful examination. Many times a driver will claim tire

failure as being the cause of the lost of control leading to

the collision. The investigator should attempt to determine

tire damage caused by the crash, as opposed to damage

existing prior to the crash or while the vehicle was being

removed for the collision scene. Roadway tire marks often

indicate tire failure prior to the crash. This can be seen

in the form of flat tire or wobble scuff marks. These scuffs

44

indicate that the deflated tire was being distorted between

the road and the rim as it rolled flat along the roadway.

Blowouts which precipitate a crash exhibit unique and

distinctive characteristics. They are indicated by those

features of the tire which appear to have been caused by the

explosive deflation of the tire. Ruptured cord fiber ends

are frayed and fuzzy, as if torn apart by a great force. A

blowout may occur before impact, during impact, or after

impact. Fresh cuts or slits in the tire are associated with

crash damage resulting from sharp metal edges coming in

contact with the tire.

VEHICLE IMPACT ORIENTATIONS

There are many variations to the impact orientation of

vehicles involved in collisions. Head-on collisions where

frontal areas of both vehicles are brought into contact with

one another may be offset by varying degrees and may be at

various angles rather than linear. During the author's 17

years in the field, with involvement in the investigation of

over 1,000 motor vehicle collisions, he has seen only one

head-on collision where the involved vehicles met perfectly

head-on, or "headlight-to-headlight."4

4 This crash involved a pickup truck and a small subcompact. Due to the truck's larger weight and higher impact speed it

pushed the smaller vehicle back in a straight line.

45

Most head-on crashes involve less than 100% overlap and

occur at offset angles to one another. Override occurs when

the frame of one vehicle does not meet the frame of the

other. The damage characteristics will determine which

vehicle overrode or underrode the other one. Motor vehicles

do not behave either as solid objects nor as elastic ones,

they behave somewhere in between. Making predictions of how

vehicles move against each other at impact, after impact,

and away from each other can be less than precise.

Head-on collisions in which one corner of the vehicle

impacts a similar corner on the other vehicle also occur.

The left front corner or left front corner orientation is

most common because vehicles travel on the right hand side

of the roadway in the United States. These crashes always

result in rotation to one or both vehicles. Each vehicle is

deflected from the point of impact towards its own side of

the road while rotating counterclockwise. For right front

impacts rotation is clockwise (as seen from above).

Side impacts between vehicle encompass a wide variety

of crash orientations and vehicle surface areas. The

investigator should attempt to determine whether the

vehicles remained engaged through part of the crash phase,

and where disengagement occurred. Engagement occurs where

the vehicles remain locked together momentarily and behave

as a single body before separating to their final rest

46

positions. At-crash and post crash movements can be

determined by tire scuff marks since wheels may be forced to

move laterally.

Rear-end impacts should also be similarly examined if

an override or underride condition existed between the two

vehicles. Fuel system integrity takes greater importance in

rear-end crashes since any failure of the system can result

in large amounts of fuel spillage, with accompanying fire

risk. Examination of the fuel tank, its mountings and

attachments, takes on greater importance in rear end

collisions.

ROLLOVERS

Unlike other crash orientations, such as head-on, side

or rear, rollovers usually consist of a series of

distributed impacts as the vehicle contacts the ground with

its sides, top, front, rear, and corner surfaces as it rolls

over. It is important to note and record the number of

impacts to which the vehicle was subjected in a rollover so

as to best determine its rollover pattern and trajectory.

Impact areas on the vehicle help determine what the

vehicle came into contact with. Often this is not the soil

but stumps, large rocks, or other irregular surface

features. In rollover configuration where the vehicle is

47

"tripped" by a curb, or other surface feature while moving

forward and yawing, or sideslipping, the roof and "A" pillar

is often subjected to the greatest impact forces. This can

be a clue to vehicle orientation as it began to rollover.

VEHICLE INTERIOR EXAMINATION

The vehicle interior can provide evidence as to how

occupants moved, what they contacted, and what damage

resulted from this contact. Contact areas may be

distinguished by tissue deposits, hair, fabric transfer,

skin oils, cosmetics, imprint of occupant features, as well

as by the deformation characteristics of contacted areas.

Occupant contact with the vehicle controls is indicated

by damage to those controls. A deformed steering wheel, bent

shift lever, or distorted dash panel control knobs indicate

occupant impact. Knowledge of the injuries sustained by the

occupants prior to examining the interior should be obtained

whenever possible. This will assist the investigator in

knowing where to look and what to look for.

Examination of the seat belts and their location can

also be helpful to the investigator. Upon examination of the

occupants physical evidence due to seat belt injuries might

be present (abrasions, bruises).

48

NOTES ____________________________________

49

NOTES ____________________________________

50

Chapter 4: MEASURING AND DIAGRAMING THE COLLISION SCENE

A collision scene diagram is of the highest importance,

as it assists the reader of the report in the visualization

of what the scene looked like. A good diagram helps the

investigating officer in testifying in court with precision

and confidence. A diagram also helps in the reconstruction

of the collision, if it becomes necessary at a later date.

Finally, collision diagrams assist in supporting probable

cause for any traffic citations or criminal charges arising

out of the collision.

WHAT TO MARK AND MEASURE

After the initial response, arrival, and scene

management responsibilities are carried out, the next step

of the investigative process is to decide what items of

physical evidence must be located, marked, and documented.

The following list is presented as a guide but it is not, by

all means, all inclusive:

1. final rest positions of vehicles and bodies; this

includes cars, trucks, bicycles, pedestrians, etc.

51

2. tire marks on the pavement or anywhere if they are

related to the collision; skid marks, scuff marks

or tire prints.

3. gouges and scratches on the road surface.

4. debris of any type, such as undercarriage, vehicle

parts, or liquids.

5. any items which were scarred or marked as a result

of the collision.

After the decision is made on what items are going to

be marked and recorded in the collision diagram, the issue

of how many marks or spots on the road surface must be

addressed. Generally, police agencies issue common spray

paint and/or lumber crayons to their officers for marking

purposes.

Bright orange or yellow paint is the most common color

as it is easily visible and located for subsequent

measuring. In the case of paint, less is usually best. There

is no need to deface the roadway with colorful "art", a

simple paint dot to locate an item is sufficient. Paint is

of little use on unpaved or dirt roadways.

52

An alternative method

on these surfaces is the use

of small flags or streamers.

These are small (4 to 6

inches) pieces of wire to

which bright streamers or

flags are attached. Coat

wire hangers and engineering

tape (found at large hardware stores) are handy materials to

make these flags. They are used at every spot where paint

would ordinarily be used and are easy to locate later,

particularly across rough or uneven fields.

Officers must remember that a discussion on marking

tools and devices is academic if they are not carried in the

police vehicle at the time they are needed. It is the

responsibility of each patrol officer to insure that all

supplies needed are in the police car at the beginning of

each shift. A professional police officer always knows what

equipment he/she has at all times.

53

Figure 7- Location and heading of a vehicle can not be established with only one measurement (left front tire, in this case)

54

After deciding what items the investigating officer is

going to want to measure, a decision has to be made on how

many marks or spots are going to be used for each item. For

vehicles, bodies, long tire marks, or large debris areas one

mark is insufficient. As an example, figure 7 shows how one

mark will not locate a vehicle's specific location and

heading as it can be facing in any direction if a single

mark is used.

55

Figure 8- Examples of how to mark different vehicles and other items at the collision scene

56

Two or more points or marks are needed for such items

as: vehicles, bodies, skid or scuff marks, and large areas

of debris. A marking on the front and rear tire (same side)

of an automobile is usually sufficient. In the case of an

articulated vehicle, such as a tractor trailer rig, they are

marked as if they were two separate vehicles. The tractor is

marked as if it was a car, so is the trailer. One mark at

the head of a body and one at the feet is sufficient to

locate it.

Straight skid marks are marked by a single spot at each

end of the skid, this is repeated for every skid mark. A

two-wheel vehicle such as a bicycle or motorcycle is marked

by placing a paint or crayon spot at each wheel as shown by

the arrows in figure 8.

On small objects, such as gouges, small scuff marks, or

small debris areas less than three feet in diameter, a

single mark or spot to the center of the object is

sufficient. On curved tire marks such as yaw marks, locate

stations along the mark at 5, 10, 15, or 20 foot intervals,

depending on the length and sharpness (radius) of the mark.

Large debris areas can be located by placing marks along its

perimeter, between 4 and 8 marks are usually sufficient.

If the collision is of such severity that a specialized

unit or investigator is going to respond to the scene, it is

best not to mark any items. The officer's role in that case

57

is to secure the scene and insure no vehicles or other

evidence are moved until the investigator arrives. This

assists the investigator in taking photographs of the

evidence in an unmarked, or "as found" condition for court

presentation.

DIAGRAMING THE SCENE

Several methods of diagramming traffic collision scenes

have been developed over the years and are in widespread

use. Some of these techniques are simple to learn and use,

while others are somewhat complex and require considerable

amounts of training.

There is very little doubt that traffic collision

investigation is, by its nature, a technical task. However,

the majority of on-scene investigative work is carried out

by regular patrol officers; this is particularly so in small

law enforcement agencies. This chapter is primarily

addressed to that group, although personnel in larger

agencies may find it useful.

With the above in mind it is easy to see why a large

number of police organizations find it necessary to teach

their officers simple yet effective collision mapping

techniques. Two of these methods are the coordinates and

58

triangulation techniques. The following discussion presents

each method, with accompanying examples.

COORDINATES

The coordinate system is based on locating any specific

spot by means of distances from a fixed reference point

(RP), along a reference line (RL). In using this method a

vital ingredient to accurate measurements is the selection

of a good reference point.

If future reconstruction of the collision is necessary,

the reference point becomes the key from which the scene is

mapped. Some examples of a good reference point are:

highways mileposts or mile markers, utility poles, fire

hydrants, culverts and bridges, intersections of roadways,

and any other manmade or natural point of a permanent

nature.

When selecting a reference point keep in mind that if

the RP is destroyed, then surveyor plans, street maps,

engineering drawings, etc., should be available which will

enable the reconstructionist to "place" the RP back at its

original location. Remember that the reference point is the

keystone from which the entire collision is measured and

drawn. A bad RP selection would make exact reconstruction of

59

the collision at a later date difficult, if not impossible,

to the degree of accuracy required.

60

Figure 9- An example of a simple field sketch using the coordinate method, with accompanying measurements table

61

To use the coordinate method, as with any other

technique, the collision investigator first prepares a field

sketch of the scene, showing all significant items. Once the

field sketch is prepared, the investigator assigns each

significant item a letter of the alphabet starting with 'A'.

After this, the investigator draws a table where all

measurements will be recorded. This table can be next to the

field sketch or on a separate page.

The table will include a legend and it should include

the following information: geographical location of the

collision, scene, weather, roadway width, complete

description of the reference point, and the investigating

officer's name and badge number. The date the diagram was

prepared and an explanation of any non-standard symbols used

can also be included here. Figure 9 shows a simple collision

scene sketch, including a measurements table and legend.

On the space above or below the legend the table will

show the collision measurement data broken down in four

separate columns: point, from '0' or RP, from roadway edge,

and an identifying description of each item or point. The

first column is simply the alphabetical listing of all

significant items marked at the collision scene. After

reaching the letter 'Z' the investigator may go on to 'AA',

'BB', etc., or an alphanumerical system such as 'A-1',

'A-2',etc., may be used.

62

The next column (From "0" or RP) will show the distance

from the reference point along the reference line to a point

abreast or perpendicular to each significant item. At each

point the investigator places a mark (paint, crayon) and

continues to measure up to the next point, without resetting

the rollatape. The '0' designation is used when the RP does

not lay directly on the reference line, such as when using a

telephone pole as the RP.

This is commonly referred to as "bringing" the RP to

the roadway edge by measurement. This distance is not part

of the table itself, but it should be shown in the legend.

It should be noted that the coordinate method should not be

used when significant items are more than 35-40 feet from

the roadway edge, as accuracy begin to suffer beyond this

distance. In such a situation a triangulation-based method

(see below) should be used.

After this process has been completed the investigator

then measures the distance from the roadway edge out to each

significant item. This information is recorded under the

"From Roadway Edge" column, next to its respective letter.

Thus, a set of measurements, or coordinates, is established

for each significant item. The last column,

"Identification", is simply a short description of each item

in the diagram.

63

Equipped with this information the investigator is now

ready to prepare a post-collision, or final position,

diagram. If the diagram is to be to scale then he will need

additional information such as radii of any curves, etc.

TRIANGULATION

Many times the final rest positions of vehicles

involved in collisions, particularly in rollovers, are too

far from any convenient roadway edge reference lines,

rendering the coordinate method somewhat inaccurate when

distances beyond 30-40 feet from roadway edge are involved.

This is a common occurrence along county and state highways,

where the high velocities involved sometimes cause the

crashed vehicle to arrive at its final rest at a

considerable distance from the roadway.

This inability to determine right angle straight lines

out to an object is due to parallax error, an apparent

change in position of an object when viewed from two or more

positions not exactly in line with the object. The greater

the distance the greater the margin of error.

64

Figure 10- Triangulation method, commonly used when evidence is at a significant distance from the roadway edge

65

At these times the triangulation method presents a much

more accurate alternative. When using triangulation each

significant item is located by establishing fixed distances

from two reference points along a reference line. In the

example given on page 52 point 'A' is established by: (1)

measuring the distance from the first reference point (RP 1)

to 'A', (2) measuring the distance from RP 1 to RP 2, and

finally (3) measuring the distance from RP 2 to 'A'.

The investigator can use as many RP's as necessary. The

only criteria to keep in mind is to select RP's that will

maintain an evenly spaced or equilateral triangle.

Each of these steps is repeated for every significant

item in the diagram. The accompanying table shows the

recommended method of documenting these measurements. Two

measurements must be obtained for each significant item.

To draw the diagram to scale a common compass, which

has been previously adjusted to the distance required using

the scale on the template, is used to draw an arc5 from each

of the RP's. The point at which they intersect is the

location of the significant item.

The methods explained above are simple, easy to learn,

and do not require a large amount of equipment or time to

use. They are accurate and will cover every collision

diagramming problem officers are called on, with minor

5 An arc is a portion of a circle, or the part between two points on a curve.

66

modifications. The coordinate method is easier to use and

can be applied to about 90 percent of all collision scenes,

and the triangulation system is available for those few

times it is required.

DRAWING THE SCALE DIAGRAM

The last step in the measuring and diagraming process

is to prepare a scale diagram. This is a representation of

the collision scene which is proportionate to the actual

scene, based on a given scale. In reality, most collisions

investigated by patrol officers will require no more than a

simple, not-to-scale, diagram showing vehicle final

positions and other physical evidence.

Scale scene diagrams are time consuming and not

necessary for most collisions, department policy and local

practices will generally dictate when a scale diagram will

be prepared. Most law enforcement agencies only require

their patrol officers to prepare a scale diagram on fatal

collisions, other require them in collisions involving

deaths and serious injury.

In order to draw an accurate and technically acceptable

scale collision diagram, the following is needed:

1. field sketch

2. field notes

67

3. table of measurements and legend information

4. state-issued diagram form, or plain paper

5. template, with 1=10 & 1=20 scales

6. compass

7. fine point pencil

The first step in drawing a scale diagram is to decide

what items are going to be included. It would be technically

possible but extremely time consuming to attempt to include

every single item at the collision scene, such as sidewalks,

poles, street signs, bushes, and other items commonly found

on or near the scene. The rule to remember is to include

those items in the diagram which are relevant to the

collision.

All roadway evidence, such as skid marks, must be

included in the drawing. The final positions of the vehicles

is of course critical to an understanding of the collision

and are also shown. The roadways involved are shown but

sidewalks and gutters are not included unless they are of

relevance, i.e., in a case of a vehicle jumping a curb and

striking a pedestrian on a sidewalk.

In drawing the diagram a template designed for

collision mapping is essential. Two templates commonly used

by investigators are the TRAFFIC TEMPLATE AND CALCULATOR

from the Traffic Institute, and the BLUE BLITZ from IPTM6.

6 The Traffic Institute, Northwestern University, P.O. Box 1409, Evanston, IL 60204 Institute of Police Technology and Management, University of North Florida. 4567 St. Johns Bluff Rd, Jacksonville, FL 32216.

68

Both templates are made out of plastic, have a nomograph for

speed calculations, include special cut-outs for drawing

cars and trucks, and have the two most commonly used scales:

1 inch = 10 feet, and 1 inch = 20 feet, or 1=10 / 1=20.

These scales are printed on either edge of the template.

The selection of scale to be used in the diagram will

determine how large the final product will be. A standard 8½

by 11 in. Arizona Traffic Accident Report Diagram form will

accommodate a collision scene of approximately 160 feet by

135 feet on a 1=20 scale, and 81 feet by 68 feet on a 1=10

scale. If a larger diagram is necessary then additional

sheets can be taped together, or a smaller scale must be

used7.

In drawing the diagram it is a good idea to show North

towards the top of the page, this is an accepted rule in map

making and should be complied with whenever possible. In

order to show intersection curves, or other roadway curves

to scale, certain measurements have to be obtained at the

scene:

1. Chord - straight line intersecting a curve at two

points (tangent).

2. Middle Ordinate (M.O.) - shortest distance from

the center of a chord out to a point on its perimeter.

7 As an example, if a 1=20 scale is not small enough to keep the diagram manageable, a 1=40 scale can be used. In this case

the same 1=20 side of the template can be used but all measurements are divided by 2 before using the template.

69

3. Radius - the distance from the center of a circle

to a point on its perimeter.

70

Figure 11- Intersection curve diagram showing the location of the chord line, middle ordinate, and tangent points

71

The first step in calculating the radius of a curve is

to measure its chord line. This is done by stretching a 100

foot tape from tangent point to tangent point (see figure

11). At the midpoint the middle ordinate is then measured

out to the curve. As an example, if the curve chord

measurement was 50 feet then the middle ordinate measurement

would be taken at the 25 foot mark.

Once the chord and middle ordinate measurements are

recorded, and roadway width measurements are also taken,

then all necessary information is available to go to the

next step in the process. A mathematical calculation, using

the radius of a curve equation, has to be completed before

using a compass to draw the curve. The equation is:

Where R= the radius of the curve C= the chord measurement M= the middle ordinate measurement 8 & 2= are constants; in a mathematical equation a constant is a value that never changes its value; i.e., in this equation 8 and 2 are always used as such.

As an example, the next series of steps will show how

the equation is worked out, using a value of 50 feet for our

hypothetical roadway curve, and 7 feet for the middle

ordinate. The final result of the equation is a radius of

the curve of 48.14 feet. In actuality, one tenth of a foot

2M+

8MC=R

2

72

is a little over one inch, which is impossible to discern in

a 1=10 or 1=20 scale diagram, so we can safely round down to

an even 48 feet.

The next step in the process of drawing the curve, now

that we know the radius, is accomplished using the template

and a common drawing compass.

Start by drawing two straight, intersecting lines as

shown in figure 12 (step #1). The "cross" should be on the

side of the page or paper that the curve is to be. This is a

2M+

8MC=R

2

27+

(8)(7)50=R

2

3.5+56

2500=R

3.5 + 44.64=R

feet 48.14=R

73

trial-and-error process sometimes, and the investigator will

improve this skill with practice.

74

Figure 12- Steps #1 through #3 used in drawing the intersection curves to scale

Figure 13- Final steps #4 and #5 in the curve drawing process

75

After deciding what scale is to be used in the diagram

(1=20 for most diagrams), the compass gap is then adjusted

to the distance calculated in the radius of a curve equation

using the template, in this case it would be adjusted to 48

feet. The sharp metal point of the compass is then placed at

the intersection of the two lines and two short arcs are

draws, intersection each line as shown in step #2.

In step #3 the sharp point of the compass is placed at

the intersection of each arc made in step #2 above, and two

additional arcs are drawn towards the side the final roadway

curve will be. Step #4 consists of placing the sharp point

of the compass at the intersection of the last two arcs

drawn, then drawing the curve from one side to the other

(step #5). The straight lines are now erased, leaving a

perfect, to scale, roadway curve.

76

Figure 14- A four-way intersection with all curves drawn to scale, prior to erasure of roadway edge lines

77

The process is completed by extending all lines and

measuring the space between the lines with the template, to

match the roadway widths at the collision scene (figure 14).

Once the curve drawing procedure is carried out three more

times, a four-way scaled intersection is finished.

Investigator should keep in mind that, although all

four curves at an intersection are generally of the same

radius, this is not always so. Measure each curve to be

sure.

Of course, not all

intersections are 4-ways or

T-bones, meaning they do not

all meet at nice, easy to

draw 90 angles on a piece

of paper without thought

given to measuring actual

angles. Many roadways join

at acute or obtuse angles8, such as the one depicted on the

figure at the right. If the investigator simply guesses and

draws free-hand the intersection on the paper, it is no

longer a scale diagram. The investigator must first

determine the angle at which one roadway meets the other.

8 An acute angle is any angle less than 90 , an obtuse angle is any angle greater than 90 .

78

In order to do this, the investigator first visually

extends the roadway edges and marks the point at which the

Figure 16- To measure the intersection angle, establish the apex and mark off a point along each side

79

two meet (apex). Once this is done then an arbitrary but

equal distance is measured and marked from the apex along

each roadway edge (A and B in figure 16). The distance

between A & B is recorded, along with the first distance

measured.

As an example, lets assume that the first distance was

40 feet and the second was 15 feet. On the diagram paper the

investigator begins by drawing a straight line across the

page, keeping in mind where the intersection is going to

come in at.

Using a compass/template and adjusting the gap to the

first distance (40 feet in this example), the investigator

then draws an arc with the sharp point of the compass at the

apex. The arc will cross the first line drawn, as shown in

figure 17 (step #1). The compass is then removed and

adjusted to the second measurement (15 feet), the sharp

point is now placed at the intersection of the straight line

and the arc. A second arc is now drawn intersecting the

first arc (step #2).

80

The next step is to connect the apex and the

intersection of the two arcs using a straight edge, this

Figure 17- Steps to draw the angled intersection to scale

81

will provide the angle of the intersection. To complete the

drawing the other two roadway edges have to be added, in

accordance with the actual roadway width measurements. A

last step required is to draw the radius of each curve. The

mathematical equation is the same one used for 90

intersections, but the process to draw the curves is

slightly different.

Once the angle intersections has been drawn to scale,

then the radius of each curve is added. To do this, measure

two parallel lines, one on each side of the angle and to the

distance of the radius obtained from the equation. Where

these lines cross, place the sharp point of the compass and

draw the curve.

This process is then repeated for the opposite curve,

using the radius calculated for that curve. Figure 18 shows

how to complete these steps.

82

Figure 18- Drawing the curve radius, to scale, of an angled intersection

83

NOTES ____________________________________

84

NOTES ____________________________________

85

Chapter 5: ESTIMATING SPEED FROM TIRE MARKS

Tire marks are one of the most important pieces of

evidence among all of the physical evidence found at the

crash site. They can not only be used to establish the

precrash path and orientation of the vehicles, point of

impact and postcrash travel, but they can be of great help

in estimating vehicle speed prior to skidding. Identifying

tire marks at the crash site is fundamental to the crash

investigation.

SKID MARKS

Estimating speed from skid marks is one area that has

perhaps been most written about, and is of primary concern

to collision investigators where law enforcement

considerations in crashes must be considered. Since skid

marks are solid physical evidence of the crash, they can be

used to estimate the speed of the vehicle at impact as well

as its speed prior to skidding.

It is unfortunate, at times, that police investigators

will clearly neglect or ignore skid marks as a source of

probable cause for speed violations. This is usually due to

poor training and lack of awareness of the speed

determination process, which is actually quite simple.

86

Any moving body possesses a certain amount of kinetic9

energy by virtue of its motion. How much kinetic energy is

involved is dependent on two factors: the mass of the body

and its velocity. The mathematical equation for kinetic

energy is:

Where Ke= kinetic energy m= mass of the body v= velocity

Through a process called formula derivation the basic

kinetic energy formula or equation above serves as the basis

for the speed-from-skid formula. A moving vehicle's work

energy is determined by multiplying the weight of the

vehicle times the drag factor10 of the road surface times

the distance it will slide over while stopping.

9 The word "kinetic" is from a Greek word meaning" motion".

10 Throughout this discussion the term "drag factor" will be used, even when referring to coefficient of friction, for

simplicity. They are not the same but their differences are only of importance to reconstructionists.

mv21=Ke 2

MV21=WfD 2

87

Mass is the same as the weight of an object divided by the acceleration of gravity, or W/g. If we change M to W/g and divide out W, it leaves:

To get V2 alone, both sides are multiplied by 2g:

In the next step of the derivation process, the g (gravity) is changed to its actual value of 32.2 feet per second2. This 32.2 is multiplied by the 2 next to it, which becomes 64.4:

The next step is to change V2 to the more conventional and familiar miles-per-hour. There are 5280 feet in a mile, and 3600 seconds in an hour. By dividing 5280/3600 the conversion factor of 1.466666667, or 1.466, is obtained:

Carrying out the square function of 1.466 yields 2.15, this value is then divided out to leave S2 by itself:

V2g1=fD 2

fD 2g=V 2

fD 64.4=V 2

fD 64.4=)1.466 x (S 2

2.15

fD 64.4=2.15

2.15 x S 2

88

The last step is to extract the square root from both sides of the equal sign (squaring a number is the inverse of the square root):

Where S= speed f= drag factor 30= constant D= distance vehicle skidded

This is the basic speed formula commonly taught and

used in determining speed from skid marks. Although it is

not necessary that the collision investigator be proficient

in the derivation process, it is always a good idea to be

able to explain, or at least have an understanding, of the

scientific origins of the basic speed-from-skid equation.

As can bee seen from the equation above, in order to

determine the speed of the vehicle at the time braking was

initiated, the investigator needs two pieces of information:

the distance the vehicle slid and the drag factor11.

There are a couple of different approaches to the drag

factor question. The investigator can refer to one of many

published tables showing a sampling of drag factors for

11 A number representing the acceleration or deceleration of a vehicle as a decimal fraction of the acceleration of gravity;

a dimension-less number representing how slippery the road-tire combination is.

fD 30=S 2

f D 30=S

89

different surfaces (more on this later) or he/she can

conduct a skid test to obtain one.

TEST SKIDS

To obtain a drag factor from a test skid, ideally the

vehicle that was involved in the collision should be used.

Since this is highly impractical, a police vehicle is an

acceptable substitution.

The test skid should be made at or near the same

location and with the same surface conditions. For maximum

safety a second police officer should be present to monitor

traffic and block the roadway while the test is being

conducted. A good, safe speed for skid tests is 35 miles per

hour. Speeds beyond this increase the safety risk and do not

yield substantially better results. If possible, a radar

unit should be used during testing to prevent errors caused

by defective speedometers.

To do the test, the police vehicle is accelerated to 40

miles per hour and the driver steps of the gas pedal,

allowing the vehicle to coast down to 35 mph. Upon reaching

35 mph the driver applies the brakes HARD (panic stop) until

90

the vehicle slides to a stop. The longest skid mark should

be measured and recorded, and a second test conducted. If

the second test yield a distance close to or similar to the

first (within 10 percent), then the test is valid.

If the collision

occurred on a roadway which

is sloped at a significant

grade, and the test skid was

conducted on the same

surface and in the same

direction, then the grade is

already accounted for.

However, if no test skid was conducted and a drag factor

table is to be used,then the percentage of grade has to be

factored into the selected drag factor.

Figure 19 shows how to obtain the road grade using a

common carpenter's level. One end of the level is place on

the roadway surface and the opposite end is placed directly

down-grade from the end that is on the road. Insuring the

bubble is level, a measurement is taken (in inches and

tenths) from the road to the bottom edge of the level. In

the example above, the run is 18 inches and the rise is 1

inch. Dividing 1 by 18 yields a grade of .05 percent.

To use this grade on the selected drag factor, if the

sliding vehicle was going uphill, the grade is added. As an

Figure 19

91

example; if the selected drag factor is .7 and the grade is

.05 percent, then .7 + .05 = .75 for drag factor. If the

vehicle was sliding while traveling downhill, the grade is

subtracted.

Once the test skid data is obtained, this information

can be used in the drag factor equation:

Where f= drag factor S2= test speed squared 30= constant d= distance test vehicle skidded

As an example, assume that a test skid yielded a skid

distance of 55 feet with a test speed of 35 miles per hour:

The drag factor for that surface, with all wheels

locked and sliding, is .74, this would be the value used as

"f" in the speed formula discussed earlier. Lets assume that

the collision vehicle skidded for 66 feet. Using the drag

factor calculated above:

30dS=f

2

.74 =f 16501225=f

55 x 3035=f

2

92

The collision vehicle was traveling at no less than 38

miles per hour at the beginning of the skid. Why do we say

"no less than" ? Simple, the vehicle collided with another

one (or an object) before coming to rest. If it had not, the

skid marks would have been longer.

It is important to remember that the speed calculated

from the speed equation is the true speed of the vehicle

only if it slides to a stop, that is, all energy is

dissipated sliding to rest. When it is not, that is, some

energy is dissipated doing crush damage then this energy is

not accounted for in the speed equation. This is why we

refer to speed from skids as a minimum speed.

DRAG FACTORS FROM PUBLISHED TABLES

There are various sources of sample drag factors for

different surfaces, to be used when the investigator is

precluded from doing a test skid. Most law enforcement

agencies do not allow police officers to conduct test skids

on anything but serious injury or fatal collision. Some

agencies do not permit test skids at all.

38mph= S1465.2= S.74 x 66 x 30=S

93

Under these circumstances the investigator can still

make a speed determination

with the careful use of a

sample drag factor from a

published table. The

Northwestern University

Traffic Template &

Calculator instructions

booklet includes a table of

drag factors, and there are

many other sources found

among collision

investigation &

reconstruction manuals.

Table 2 shows a sample of common drag factor ranges

found among the many tables. Since using a table involves a

range of values, it is always better to conduct a skid test

if possible.

A last note on speed from skid marks: if the collision

vehicle was braking at less that 100% efficiency (less than

all four wheels braking), an adjustment has to be made to

the drag factor before it is used in the speed equation. A

discussion on this topic is beyond the scope of this manual.

Investigators suspecting less than all four tire braking

Average Sliding Drag Factors for Passenger Car Tires __________________________ Surface Drag Factor Asphalt Concrete, dry .65 - .90 ------------------------- Asphalt Concrete, wet .45 - .70 ------------------------- Gravel loose to packed .45 - .70 ------------------------- Road, w/ snow .30 ------------------------- Ice .15 -------------------------

Table 2

94

should consult with someone with advanced training,

preferably a collision reconstructionist, for assistance.

THE TEMPLATE NOMOGRAPH

A quick way to calculate speed from skid distance,

without utilizing the mathematical equation, is available

using either the Northwestern template or IPTM's Blue Blitz

template. Both templates have a set of scales printed down

the center of each instrument.

To use nomograph to

calculate speed; first a

drag factor is selected. The

example on figure 20 uses a

.7 for drag factor and 60

feet for the distance of the

skid. Using a straight edge

and connecting the drag

factor scale (right scale) with the distance scale (left

scale) yields a speed of approximately 35 mph (center

scale). The nomograph is an easy to use instrument and very

helpful to those investigators who have difficulty with the

equation. It can also serve as a "quick and dirty" speed

estimation method at the collision scene.

Figure 20

95

CRITICAL SPEED (YAW) MARKS

In Chapter 2 a discussion of critical speed marks and

their value to collision investigation was presented.

Critical speed marks, or yaw marks, are tire marks left on

the roadway by wheels that are sliding and rolling at the

same time. they are made by the tire sidewall and are a

result of the driver oversteering the vehicle.

A common occurrence where yaw marks are found is in

rollovers involving drunk drivers. The intoxicated driver

falls asleep and allows the vehicle to drift off the

roadway. When the driver suddenly awakens he/she jerks the

wheel to the left, launching the vehicle into a sideslipping

maneuver. This brings the vehicle across opposite lane of

traffic, off the shoulder and completely off the roadway,

usually flipping in the process. The telltale yaw marks can

be used to estimate the vehicle speed at the beginning of

the maneuver.

In order to calculate the speed at which the vehicle

began to sideslip, the investigator must first know the drag

factor (from skid test or table) and the radius of the curve

followed by the vehicle. The mathematical equation used in

calculating the radius has already been discussed in Chapter

4. Figure 21 shows how to obtain the chord and middle

ordinate (m.o.) of the yaw mark.

96

When measuring the chord, the beginning of the tape

measure is placed at the beginning of the yaw mark, and a

Figure 21- How to obtain the chord and middle ordinate (M.O.) measurements from a yaw mark

97

chord of 100 feet is generally used. When measuring the

middle ordinate particular attention must be given to

obtaining a precise measurement, as the equation is

extremely sensitive to middle ordinate changes.

Once the radius of the curve has been calculated then

this value can be used in the critical speed equation:

Where S= speed 15= constant R= radius f= drag factor

As an example, if a critical speed yaw mark left by a

vehicle that subsequently ran off the roadway and overturned

had a radius of 335 feet, and the drag factor was .7 for

that road surface:

The speed of the vehicle when it began to sideslip was

59 miles per hour.

COMBINED SPEEDS

f R 15=S

59mph = S3517.5= S.7 x 335 x 15=S

98

Sometimes vehicles skid across two different surfaces

before stopping or colliding with another vehicle or object.

Using the same drag factor for both distances is a common

approach, it is also wrong. To approach this problem the

total distance of the slide must be divided and each segment

must be calculated using the basic speed equation.

Upon completion of the step the investigator will have

two different and separate speeds. They are not added !

These speeds, speed #1 and speed #2 must now be used in the

combined speed equation:

Where Sc= combined speed S12= speed #1 squared S22= speed #2 squared

As an example, if the two speed calculations resulted

in 40 mph for speed #1 and 60 mph for speed #2:

S+S=S 22

21c

mph 72 =S 3600+1600=S 60+40=S cc22

c

99

NOTES ____________________________________

100

NOTES ____________________________________

101

Chapter 6: THE COLLISION REPORT NARRATIVE

The report narrative segment of the police traffic

collision investigation is the final topic of this manual.

Forms used for reporting collisions are covered in other

manuals so they will not be discussed here. The collision

report narrative is where the investigator reports the

results of the investigation, his observations and findings,

and arrives at a conclusion on cause and contributing

factors in the collision.

The author of this manual has seen numerous collision

investigation report narrative formats, some are better than

others, but there is no one format that is always best. The

selection of narrative style is usually left to the

department employing the officer, so extensive discussions

on which format is best are somewhat academic.

For those officers whose agencies do not regulate

report narrative format and style, two different styles will

be presented as examples. Neither is truly "better" than the

other, but they are different. Investigators should look at

both and use whichever styles suits them.

102

EXAMPLE #1 FACTS: On 10/9/XX at 1640 hours I was dispatched to the intersection of 11th street and Palo Verde street, regarding an injury collision. Myself and Officer Jones arrived at 1643 hours. I saw a young boy, later known to me as Jimmy Smith (7 y.o.a.), laying on the roadway at the intersection. He was being treated by paramedics. I also saw a 1987 Dodge Coupe (AZ Lic. ABC-123) stopped on the roadway approximately 45 feet south of the boy. There was a red boy's bicycle under the Dodge (see attached diagram). I secured the scene by blocking the intersection with traffic cones. I located the driver of the Dodge, Mary Maple, standing on the sidewalk near the scene. Mrs. Maple provided me with her Arizona driver's license, and I interviewed her regarding the collision (see STATEMENTS). I also met witness J. Leno, who also provided a statement. At approximately 1650 hours the victim was transported to General Hospital by Arizona Ambulance. I took photographs of both vehicles and all roadway evidence. Mary Maple was allowed to drive her vehicle home, and the bicycle was turned over to Jimmy Smith's father, John Smith. I continued with my investigation by marking several roadway scrapes, skid marks, and the final position of the victim and the Dodge. I took all required measurements with assistance of Officer Jones and cleared from the scene at approximately 1730 hours. LOCATION: 11th Street and Palo Verde, Cactus Cove, AZ SCENE: Day time, weather was dry and clear, 11th street is an asphalt paved roadway in a north-south alignment, two lanes with stripped centerline, there is a stop sign for all traffic approaching 11th street on Palo Verde. There was a small (2 x 3 inch) red paint scraping near the left front corner of the Dodge bumper, no other damage visible. EVIDENCE: Driver/witness statements Officer's Observations

103

STATEMENTS: Mr. J. Leno told me he was watering his lawn at 1234 Palo Verde (southwest corner of intersection) when he saw the Smith boy ride his bicycle into the intersection without stopping. Leno then heard the car's brakes and suddenly the car appeared, striking the boy. He estimated the car's speed at approximately 25 mph (statement attached). Mrs. Maple told me she was southbound on 11th street, at approximately 25-30 miles per hours. She stated she is always cautious in this area as there are always children playing and riding bikes. She said that the Smith boy darted into the intersection and she didn't have time to stop(statement attached). INJURIES: The victim sustained a broken left leg, deep lacerations to the forehead,minor contusions, and a possible concussion. He was admitted to General Hospital. OPINIONS/CONCLUSIONS: The Dodge left skid marks for a distance of 67 feet, starting approximately 20 feet prior to impact. Using a drag factor of .7 yields a speed of approximately 37 miles per hour, in a 25 mph posted zone. The bicycle rider is obligated, by law, to stop at the sign prior to entering the intersection. Based on available evidence it is my opinion that the collision was caused by the pedalcyclist's failure to yield to the Dodge. The Dodge was exceeding the posted speed limit by 17 mph, this was a contributing factor to the severity of the injuries. RECOMMENDATIONS: The driver of the Dodge was cited for speed.

104

EXAMPLE #2 INITIAL OBSERVATION Upon arrival at the scene, I observed a vehicle (traffic unit #1) stopped on the south side shoulder of the on-ramp to I-40 milepost 289G. A pedestrian was lying in the middle of the roadway being attended by local ambulance personnel at that location. there were several items of personal apparel in the roadway prior to the pedestrian as well as some under-carriage debris. ENVIRONMENTAL AND ROAD FACTORS The location of the collision is a one-way on-ramp eastbound at milepost 289G. The roadway is constructed of asphalt surface and was full of defects. TRAFFIC UNIT INFORMATION Traffic unit #1 sustained contact damage to the grill, hood and windshield that was consistent with collision with a pedestrian. Several pieces of fabric were observed in the grill that appeared consistent with clothing worn by traffic unit #2. Traffic unit #2 is a pedestrian that sustained major trauma to the head, neck, and both legs. he was transported to Jim's Hospital by the rural ambulance company. WITNESS INFORMATION There were no known witnesses to the collision. INVESTIGATION Investigation revealed that traffic unit #1 was eastbound on the on-ramp to I-40 at milepost 289G accelerating onto the interstate. The driver stated that as he reached the top of the ramp, he observed a pedestrian walking in the roadway. He attempted to stop his vehicle by applying his brakes but was unable to do so. His vehicle struck the pedestrian from the rear. The pedestrian rotated onto the hood of vehicle #1 and then into the windshield. Evidence at the scene was consistent with the driver of traffic unit #1's statement and is documented on the attached diagram.

105

NOTES ____________________________________

106

NOTES ____________________________________

107

Appendix

108

EQUATIONS FOR THE TRAFFIC COLLISION INVESTIGATOR DEFINITIONS a = acceleration or deceleration rate, in feet per second per second (fps2) C = chord D = distance f = acceleration or deceleration factor, drag factor or coefficient of friction g = Acceleration due to gravity (32.2 feet per second2) M = middle ordinate R = radius of a curve S = speed, in miles per hour t = time, in seconds V = Velocity, in feet per second ____________________________________________________________ Velocity to travel a known distance in a known time:

Distance traveled at a constant velocity and a known time:

Time to travel a known distance at a constant velocity:

tD = V

t V = D _

VD= t

109

Acceleration or deceleration factor when speed and distance are known:

Acceleration or deceleration rate when acceleration/deceleration factor is known:

Basic speed formula, decelerating to a stop when distance and drag factor are known:

Combined speed:

Converting speed (MPH) to velocity (FPS):

Converting velocity (FPS) to speed (MPH):

Critical speed of curve, or path of center of mass when radius and drag factor are known:

D 30

S= f2

_

f g= a _

f D 30= S __

......S + S + S=S 23

22

21

1.47 S= V _

1.47V= S

110

Distance to accelerate or decelerate to a or from a stop, when speed and acceleration/deceleration factor are known:

Radius of a curve when chord and middle ordinate are known:

f R 15= S __

f 30

S= D2

_

2M+

M8C= R

2

_

111

TRAFFIC COLLISION TERMINOLOGY ACCELERATION: time rate of change of velocity; change of velocity divided by time; a vector quantity measured in feet per second per second (fps2) or expressed as a decimal fraction of the acceleration of gravity (32.2 fps2). ACCIDENT (COLLISION): occurrence in a sequence of events that usually produces unintended death, injury, or property damage. The term 'collision' has gain wider acceptance as a more accurate term for what used to be referred to as an accident. APEX: the point at which two sides of an angle meet or cross. AREA OF IMPACT: the place on the roadway or ground closest to the first contact between colliding objects. ARC: part of a curve; especially part of a circle, between two points on a curve. BERM: see shoulder. BRAKING DISTANCE: the distance through which brakes are applied to slow a vehicle; the shortest distance in which a particular vehicle can be stopped by braking from a specified speed on a particular surface; the distance from brake application to collision. BRAKING SKID MARK: see skid mark. CENTRIFUGAL FORCE: the force of a body in motion which tends to keep it continuing in the same direction rather than following a curved path. CHORD: a straight line connecting the ends of an arc or two points on a curve. COEFFICIENT OF FRICTION: a dimension-less number representing the resistance to sliding of two surfaces in contact; the drag factor of a vehicle or other object sliding on a roadway or other surface which is level. CONTACT DAMAGE: damage to a vehicle resulting from direct pressure of some foreign object in a collision or rollover. It is usually indicated by striations, rub-off of material, or puncture. Compare with induced damage.

112

CONTROLLED FINAL POSITION: a final position reached because of conscious effort of some person to modify the motion of a traffic unit after a collision. COORDINATE: a method of locating a spot in an area by measurements along and at right angles to a reference line or by measurements of the shortest distances to each of two intercepting reference lines. Compare with triangulation. CRITICAL SPEED: the speed at which centrifugal force of a vehicle following a specific curve exceeds the traction force of the tires on the surface; a velocity above which a particular highway curve could not be negotiated by a vehicle without yaw. CRITICAL SPEED MARKS: see yaw marks. CROOK: an abrupt change of direction of a tire mark due to collision forces. See offset. DEBRIS: loose material strewn about the road as the result of a traffic collision; dirt, liquids, vehicle parts, and other materials from the involved traffic units. DECELERATION: rate of slowing; negative acceleration. DISENGAGEMENT: see last contact. DRAG FACTOR: a number representing the acceleration or deceleration of a vehicle or other body as a decimal fraction of the acceleration of gravity. When a vehicle slides with all wheels locked, the drag factor is the same as the coefficient of friction. ENERGY: ability to do work or produce an effect such as damage; a unit of force operating through a unit of distance; half the mass or weight times velocity squared; measured in foot-pounds (ft-lb). FINAL POSITION: the location of a vehicle or body when it comes to rest after a collision; final positions may be controlled or uncontrolled. FIRST CONTACT: the initial touching of objects in a collision; the place on the road or ground where this touching occurs. FIRST HARMFUL EVENT: the first occurrence in a traffic collision that results in appreciable damage or injury.

113

FLIP: the movement of a vehicle, without touching the ground, from a place where its forward velocity is suddenly stopped by an object such as a curb or furrow-in below its center of mass with the result that the ensuing rotation lifts the vehicle off the ground. A flip is usually sidewise but if it is endwise, it is a vault. FOGLINE: the solid white line that separates the drive lanes from the shoulder/berm area. FURROW: a channel in loose or soft material, such as soil or dirt, made by a skidding or scuffing tire or some other part of a moving vehicle. GAP SKID: a braking skid mark which is interrupted by release and reapplication of brakes or which terminates by release of brakes before collision. Compare with skip skid. GOUGE: a pavement scar deep enough to be easily felt with the fingers. GRADE: the change in elevation in unit distance in a specified direction along the center line of a roadway or the path of a vehicle; the difference in level of two points divided by the level distance between the points. HIGHWAY: the entire width between the boundary lines of every way publicly maintained when any part thereof is open to the use of the public for purposes of vehicular travel. IMPRINT: a mark on the road made without sliding by a rolling tire. INDUCED DAMAGE: damage to a vehicle other than contact damage, often indicated by bending, braking, and distortion. Compare with contact damage. INTERSECTION: when two or more roadways cross or connect, the area contained within the extension of curb lines, or, if none then the lateral roadway boundary lines is defined as the intersection. KINETIC ENERGY: the amount of energy represented by a moving body; half of the mass times the square of the velocity. LAST CONTACT: the final touching of objects in a collision before separation. MAXIMUM ENGAGEMENT: greatest penetration of one body, such as a vehicle, by another during collision; moment of greatest force between objects in a collision.

114

MIDDLE ORDINATE: the perpendicular distance between an arc and its chord at the middle of the chord. NOMOGRAPH: a graph on which three or more scales are arranged so that a straight line drawn through values on any two will cross the third at a corresponding value. OFFSET: see crook. RADIUS: the distance from the center of a circle to a point on its perimeter (circumference); the distance from a point on an arc to the center of the circle of which the arc is part. REFERENCE LINE: a line, often the edge of a roadway, from which measurements are made to locate spots, especially spots along a roadway. REFERENCE POINT: a point from which measurements are made to locate spots in an area; sometime the intercept of two reference line; RP. ROAD: the part of a trafficway which includes both the roadway, which is the traveled part, and any shoulder or berm along the roadway. ROADWAY: that portion of the highway improved, designed, or ordinarily used for vehicular travel, exclusive of the berm and shoulder. ROLLOVER: a situation where the vehicle rolls at least 90 degrees. The term rollover is also sometimes used to describe a pitchover (vault). SCRAPE: a broad area of a hard surface covered with many scratches or striations made by a sliding metal part without great pressure. SCUFF MARK: a friction mark on a pavement made by a tire which is both rotating and slipping. SHOULDER: that portion of the road contiguous with the roadway for accommodation of stopped vehicles, for emergency use, and for lateral support of the roadway structure. SKID MARK: a friction mark made on a pavement by a tire that is sliding without rotation.

115

SKIP SKID: a braking skid mark interrupted at frequent regular intervals; the skid mark made by a bouncing wheel on which brakes keep the wheel from turning. TRAFFIC: pedestrians, ridden or herded animals, vehicles, street cars, and other conveyances either singly or together while using any highway for purposes of travel. TRAFFIC UNIT: an element of traffic; a person using a trafficway for travel or transportation; vehicle, pedalcycle, pedestrian. TRAFFICWAY: see highway. TRIANGULATION: a method of locating a spot in an area by measurements from two or more reference points, the locations of which are identified for future reference. UNCONTROLLED FINAL POSITION: a final position reached by a traffic unit after a collision without conscious human intervention. VAULT: an endwise flip. VEHICLE: every device in, upon, or by which any person or property is or may be transported or drawn upon the highway, excepting devices moved by human power or used exclusively upon stationary rails or tracks. VELOCITY: time rate of change of position in which direction as well as rapidity is an element; distance divided by time if velocity is constant. YAW MARK: see scuff mark